text: The effect was first observed by Jencks and Carriuolo in 1960 in a series of chemical kinetics experiments involving the reaction of the ester p-nitrophenyl acetate with a range of nucleophiles. Regular nucleophiles such as the fluoride anion, aniline, pyridine, ethylene diamine and the phenolate ion were found to have pseudo first order reaction rates corresponding to their basicity as measured by their pK. Other nucleophiles however reacted much faster than expected based on this criterion alone. These include hydrazine, hydroxylamine, the hypochlorite ion and the hydroperoxide anion.